1. Technical Field of the Invention
The present invention relates in general to Code Division Multiple Access (CDMA) communications in cellular radio communications systems, and in particular to interference cancellation in multiple. access communications systems.
2. Description of Related Art
To enable simultaneous communications by multiple users of a cellular system, some type of multiple access technique must be used. In the past, most cellular systems have used either Frequency Division Multiple Access (FDMA), wherein each active call is allocated to a specific frequency, or Time Division Multiple Access (TDMA), wherein each user is allocated a particular time slot and frequency. For the third generation of cellular systems, however, Code Division Multiple Access (CDMA) has been selected as the primary multiple access technique. In particular, Wide-band CDMA (WCDMA) will be used in the majority of the third generation systems.
In CDMA systems, a number of users are allocated to the same frequency. To distinguish among signals for different users, each user is assigned a pseudo-noise spreading sequence, which a transmitter uses to spread the narrow-band information signal to a wide-band signal. To detect a CDMA signal, a conventional receiver despreads the received signal using the same spreading sequence that was used by the transmitter. This despreading concentrates the wide-band signal back to a narrow-band signal. Because the wide-band signal includes information for multiple users, however, the other information signals contained in the wide-band signal tend to interfere with the quality of the despread signal for a particular user. As a result, some form of interference cancellation is typically used to improve performance.
One type of interference cancellation uses knowledge of the spreading codes for all of the various users as well as estimated values of the channel characteristics and data symbols for all of the interfering signals. This type of interference cancellation can be implemented using multi-user detection algorithms, such as parallel subtractive interference cancellation. Multi-user detectors are most frequently found in the base stations of cellular systems because all of the required information is generally available in the base stations.
A second type of interference cancellation can be implemented using single-user detectors, which do not have knowledge of the actual spreading sequences used for interfering signals. Only the modulation method and spreading factor are assumed to be known. Single-user detectors are most frequently found in mobile stations because detailed information about the spreading codes or data symbols of the interfering signals is not typically available to the mobile stations. Moreover, single-user detectors are appropriate for mobile stations because they are generally less complex, in terms of the amount of processing resources needed, than multi-user detectors. Because of this reduced complexity, which makes single-user detectors easier to implement, it is sometimes desirable to use single-user detectors in the base stations as well. Interference cancellation in single-user detectors is based on adaptive algorithms that utilize the cyclo-stationary property introduced by the use of short spreading sequences. Accordingly, these adaptive single-user interference cancellation algorithms can effectively cancel interference without knowing the spreading sequences of the interfering signals.
In existing CDMA systems, the short spreading sequence has a periodicity equal to the symbol time. In other words, the length of the spreading sequence is varied to match the spreading factor. Under the WCDMA standard, however, the short spreading sequences, or scrambling codes, have a fixed length of 256 chips regardless of the spreading factor used to transmit information. Thus, the length of the spreading sequence is only equal to the symbol time when the spreading factor of the data symbols is 256. If the spreading factor is lower (i.e., the data rate is higher), the cyclo-stationary property of the interfering signals is partially destroyed. As a result, existing single-user interference cancellation techniques are not effective in connection with communications using the WCDMA standard.
Moreover, users in the WCDMA environment can be multi-rate users (i.e., the spreading factor for such users can change frequently depending upon the type of application, channel quality, and the like). At any given time, however, different users in a WCDMA system are not necessarily using the same spreading factor. Consequently, interfering signals from other users might have different spreading factors than the user of interest. These problems can further reduce the effectiveness of conventional interference cancellation techniques in the WCDMA environment.
Another problem with many existing adaptive interference cancellation techniques is that they are used to both adapt the receiver to cancel interference and to follow the variations of the radio channel. Because fading on the radio channel can occur very quickly relative to changes in the interfering signals, the adaptation of these interference cancellation techniques often has difficulty following the radio channel variations. Moreover, in addition to handling fading, the adaptive receiver must be able to receive and process signals from multiple antennas and signals propagated through multi-path channels.
There is a need, therefore, for a system and method for canceling interference when the spreading factor used for a particular transmission differs from the length of the short scrambling code for that transmission. The system and method should also be capable of processing transmission signals that are received on multiple antennas and from multiple propagation paths. Furthermore, the system and method would also enable the interference cancellation to adapt to relatively fast variations in the radio channel and to the comparatively slower variations in interference caused by other users"" signals. Preferably, the system and method would account for changes in a user""s own spreading factor and would provide effective interference cancellation when the spreading factor of interfering users differs from the user""s own spreading factor.
The present invention comprises a method and system for canceling interference in a code division multiple access (CDMA) transmission. In one aspect of the invention, multiple filter sets are used to recover a signal encoded in the transmission, wherein the signal has a spreading factor that differs from the length of a spreading sequence used to encode the signal. In another aspect of the invention, the filter or filters used to recover a signal encoded in the transmission are adapted using a normalized adaptation procedure.
In accordance with one embodiment of the invention, there is provided a method in which a signal encoded in a CDMA transmission is transmitted on a particular channel. The signal includes a plurality of symbols, each having an associated spreading factor, and is spread using a particular spreading sequence. In this embodiment, the length of the spreading sequence differs from the spreading factor associated with at least one of the symbols. Upon receiving the CDMA transmission, it is filtered using a number of despreading sequences to recover the encoded signal. Each of the despreading sequences corresponds to a particular portion of the spreading sequence. Accordingly, each symbol in the received CDMA transmission is despread using one of the despreading sequences that corresponds to the portion of the spreading sequence that was used to spread the symbol. Each despreading sequence is further adapted to cancel interference associated with its corresponding portion of the spreading sequence. Generally, such interference is caused by other signals encoded in the CDMA transmission. Preferably, the adaptation step is performed after each filtering operation to generate an updated despreading sequence for use in the next filtering operation.
In accordance with another embodiment of the invention, interference cancellation is implemented in a system that includes one or more antennas for receiving a CDMA transmission that includes a signal encoded by a particular spreading sequence. The system includes an interference cancellation adaptation unit for adapting each of a plurality of despreading vectors for purposes of canceling interference in the received CDMA transmission. In addition, the system includes one or more filtering units for despreading the received CDMA transmission using the adapted despreading vectors to recover the encoded signal. Each filtering unit comprises a plurality of despreading filters, each of which uses one of the adapted despreading vectors to despread a portion of received CDMA transmission. In particular, the specific adapted despreading vector that is used is the one that corresponds to the part of the spreading sequence used to encode the signal.
In accordance with another embodiment of the invention, a CDMA transmission, in which a signal is encoded using a particular spreading sequence, is transmitted on a specific channel. The CDMA transmission is received and filtered using at least one despreading sequence to recover the signal. Subsequently, the at least one despreading sequence is adapted for purposes of canceling interference caused by other signals in the CDMA transmission. The adapted despreading sequences are further normalized to generate despreading sequences for use in a subsequent filtering step.
As an additional alternative, an error vector can be calculated for each despreading sequence for use in adapting the despreading sequences. Preferably, each such error vector is normalized and then used to calculate the corresponding adapted despreading sequence. Furthermore, the despreading sequences can also be adapted to account for radio channel variations, wherein the adaptation is performed according to an amount of energy and a phase associated with portions of the received CDMA transmission.
In yet another embodiment, there is provided an interference cancellation system having at least one antenna for receiving a CDMA transmission that includes therein a signal encoded by a spreading sequence. The system includes an interference cancellation unit for adapting at least one despreading vector to cancel interference in the received CDMA transmission. The interference cancellation unit also normalizes the adapted despreading sequences to generate updated despreading sequences for use in subsequent filtering. The system further includes one or more filtering units for despreading the received CDMA transmission using the normalized, adapted despreading vectors to recover the encoded signal.